Bidirectional Power Transfer between Grid and Electric Vehicle Batteries

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www.usn.no

Faculty of Technology, Natural sciences and Maritime Sciences

Campus Porsgrunn FMH606 Master's Thesis 2020

Electrical Power Engineering

Bidirectional Power Transfer between Grid and Electric Vehicle Batteries

AC

DC DC

DC

Power Grid Electric Vehicle

EV Charging Station

Okbe Kifle Habte

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www.usn.no

Course: FMH606 Master's Thesis, 2020

Title: Bidirectional Power Transfer between Grid and Electric Vehicle Batteries Number of pages: 79

Keywords: Electric vehicle (EV), bidirectional (two-way) power transfer, AC-DC converter, DC-DC converter, EV bidirectional charger, Vehicle-to-grid (V2G)

Student: Okbe Kifle Habte

Supervisor: Kjetil Svendsen External partner: -

Availability: Open

Summary:

In the last decades, the significant growth of electric vehicle (EV) is promising an alternative to solve the concern about fossil fuel and global warming. However, as the number of EVs increasing significantly, it can cause overload in the existing distribution network of the power system. The vehicle- to-grid (V2G) technology is recognized as the best alternative to mitigate the stress in the electric grid by providing ancillary service and power balancing in the power system. The tasks covered in this thesis are:

Different effective ways of EV charging are reviewed, and their advantage and disadvantages are addressed. A literature review of common bidirectional AC-DC and DC-DC converters are carried out.

Several converter topologies can be used to implement the bidirectional EV chargers. It is found that the two-level three-phase AC-DC converter and either half-bridge buck/boost for a non-isolated converter or a dual active bridge for an isolated converter in DC-DC converter are leading topologies.

For safe and reliable power transfer between EV and the grid bidirectionally, the available bidirectional charging standard is investigated. Right now, the only bidirectional charger accepted as standard is the CHAdeMO with DC charging. Finally, a two-stage bidirectional EV charger with and without galvanic isolation is developed and simulated in MATLAB/Simulink. The first stage is a three-phase AC-DC converter, and the second stage is modeled with both half-bridge non-isolated DC-DC converter and dual active bridge isolated DC-DC converter. All the converters transfer power bidirectionally. The simulation results showed that the two-way power transfer with the proposed bidirectional EV charging station models is feasible.

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Preface

This thesis has been done in the last semester to complete the two years Master’s program in Electrical Power Engineering at the Univerisity of South-Eastern Norway (USN). Kjetil Svendsen proposed and supervised the thesis, “Bidirectional Power Transfer between Grid and Electric Vehicle Batteries.” The main tasks in this thesis are literature review on converter technology for bidirectional EV chargers, the effective way of charging EV from environmentally friendly energy sources, and investigate the available standard bidirectional EV chargers. At last, to develop a simulation model of a bidirectional electric vehicle charger in two parts separately with and without galvanic isolation in MATLAB/Simulink.

I would like to express my deepest gratitude to my thesis supervisor associate professor Kjetil Svendsen for his guidance, support, and especially for giving me unlimited time.

Porsgrunn, 15 May 2020 Okbe Kifle Habte

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Contents

Nomenclature

Symbol Unit Description

AC - Alternating current

BPT Bidirectional power transfer

CCS Combined Charging System

DC - Direct current

DERs - Distributed energy resources

EV - Electric vehicle

EVSE - Electric vehicle supply equipment

f Hz Frequency

𝑓_𝑠 Hz Frequency of the transformer

G2V - Grid to vehicle

HV - High voltage side of the transformer 𝑖_𝑎(𝑡), 𝑖_𝑏(𝑡), 𝑖_𝑐(𝑡) A Utility time-varying phase currents

𝑖_𝐷𝐶 A Current in the DC side of DC-bus

I_rms A RMS utility phase current

ICE - Internal combustion engine

𝐿_𝑙𝑘 H Transformer linkage inductance

LV - Low voltage side of transformer

𝑁 - Transformer turns ratio

𝑃 W Real power in AC side of the converter

𝑃^∗ W Reference real power

𝑃_𝑠 W Real power transfer in the converter

PV Photovoltaic

Q Var Reactive power in AC side of the converter

𝑄^∗ Var Reference reactive power

PWM - Pulse width modulation

SOC - State of charge

𝑉₁ V Transformer low voltage

𝑉₂ V Transformer high voltage

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Nomenclature

𝑣_𝐷𝐶 V Voltage in the DC side of AC-DC

V_rms V RMS utility phase voltage

converter

t s Time

𝜑_𝑖 radian Phase angle

𝜑_𝑖^∗ radian Reference phase angle

𝜑 radian The phase shift angle between the square

wave in both terminals of the transformer

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List of Figures

Figure 2.1: Stand-alone solar energy EV charging station [22]. ... 17

Figure 2.2: Giraffe 2.0 hybrid (solar-wind) stand-alone power supply system [27]. ... 18

Figure 3.1: A bidirectional single-phase ac-dc converter in a load connected to dc microgrid and the grid [38] ... 22

Figure 3.2: Control scheme for the simplified PWM switching [38]. ... 23

Figure 3.3: Three-phase two-level H-bridge bidirectional three-phase AC-DC converter [38]. ... 25

Figure 3.4: Three-phase four-leg bidirectional AC-DC converter [37]. ... 26

Figure 3.5: Three-phase three-level neutral point clamp AC-DC converter [34]. ... 26

Figure 3.6: Single-stage three-phase bidirectional AC-DC converter [6]. ... 27

Figure 3.7: Cascade bidirectional DC-DC converter topology[44]. ... 28

Figure 3.8: Unidirectional buck and boost converters. ... 28

Figure 3.9: Buck-boost bidirectional DC-DC converter [46]. ... 29

Figure 3.10: Bidirectional three-level DC-DC converter [47]. ... 29

Figure 3.11: Bidirectional full-bridge DC-DC converter based on two voltage-fed [49]. ... 30

Figure 3.12: Half-bridge bidirectional DC-DC converter [54]. ... 32

Figure 3.13: Centertapped push-pull bidirectional DC-DC converter [55]. ... 32

Figure 4.1: Some types of on-board and off-board connectors [57]. ... 34

Figure 4.2: Wallbox bidirectional charger[78]. ... 40

Figure 5.1: A simplified two-stage bidirectional EV charger without galvanic isolation. ... 42

Figure 5.2: Half-bridge single-phase bidirectional AC-DC converter... 43

Figure 5.3: When the phase shift is 0° in a sine wave, no current flow. ... 44

Figure 5.4: AC voltage and current under rectifier mode with a 1° phase angle... 45

Figure 5.5: AC voltage and current under inverter mode with a 1° phase angle. ... 45

Figure 5.6: AC voltage and current under inverter mode with a 2° phase angle. ... 46

Figure 5.7: AC voltage and current for both rectifier and inverter mode. ... 46

Figure 5.8: Bidirectional DC-DC converter... 47

Figure 5.9: Input and Output voltages of the bidirectional DC-DC converter in buck mode. . 48

Figure 5.10: Input and Output voltages of the bidirectional DC-DC converter in Boost mode. ... 49

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List of Figures

Figure 5.13: AC side voltage and current in phase A during charging... 52

Figure 5.14: Three-phase grid voltages during charging. ... 52

Figure 5.15: Three-phase grid currents during charging. ... 53

Figure 5.16: Battery DC charging current. ... 53

Figure 5.17: Center-taped DC link voltage during rectifier mode. ... 53

Figure 5.18: Active and reactive charging power. ... 54

Figure 5.19: The three-phase current and voltage in phase B during charging. ... 54

Figure 5.20: AC voltage and current converted by the bidirectional AC-DC converter during discharging. ... 55

Figure 5.21: AC voltage generated by the inverter, AC grid voltage, and AC current (50 A). ... 55

Figure 5.22: AC voltage generated by the inverter, AC grid voltage, and current (25 A). ... 56

Figure 5.23: Three-phase AC voltage produced by the bidirectional AC-DC converter during discharging. ... 56

Figure 5.24: Three-phase AC current in discharging mode. ... 57

Figure 5.25: DC discharging current at phase angle 10°. ... 57

Figure 5.26: DC discharging current at phase angle 5°. ... 57

Figure 5.27: Center-taped DC link voltage. ... 57

Figure 5.28: Active and reactive power injecting from the battery to the grid. ... 58

Figure 5.29: AC voltage and current for rectifier nad inverter mode in phase A. ... 58

Figure 5.30: Charging and discharging DC current. ... 59

Figure 5.31: Simulation model proposed for EV charger without galvanic isolation. ... 59

Figure 6.1: Bidirectional dual active bridge DC-DC converter. ... 63

Figure 6.2: Low voltage side AC square voltage. ... 64

Figure 6.3: High voltage side AC square voltage. ... 64

Figure 6.4: Average charging DC current in the LV and HV side𝜑 = 2.16°. ... 65

Figure 6.5: Average discharging DC current in the LV and HV side 𝜑 = 3.6°. ... 65

Figure 6.6: Average discharging DC current in the LV and HV side φ=5°. ... 65

Figure 6.7: A two-stage bidirectional EV charger with galvanic isolation... 66

Figure 6.8: DC-link voltage during charging. ... 67

Figure 6.9: Average charging DC current in the high and low voltage side of the DC converter. ... 68

Figure 6.10: AC voltage and current when power flows from the grid to the EV battery. ... 68

Figure 6.11: DC-link voltage during discharging. ... 69

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List of Figures Figure 6.12: Average discharging DC current in the high and low voltage side of the DC-DC

converter. ... 69

Figure 6.13: AC voltage and current when power transfer to the grid. ... 69

Figure 6.14: HV and LV side voltages and inductor current displaying soft switching. ... 70

Figure 6.15: Simulation model proposed for EV charger with galvanic isolation. ... 70

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Introduction

1 Introduction

Concern about climate change and its solutions are among the biggest concerns of governments, media, including individual activists all around the world. The problem of global warming is due to the increase in greenhouse gas emissions by human activities. A large part of greenhouse gas is produced from fossil fuel (coal) energy sources and emission from conventional vehicles with an internal combustion engine (ICEs). Now, almost 30% of greenhouse gas emissions in the USA are produced by the transportation sector. Transportation includes cars, ships, trains, trucks, and planes [1]. Moreover, a large number of EVs are already on china’s roads; however, one research study [2] discovered that EVs could account for more than twice the emission of the traditional cars powered by gasoline because of the fossil fuel dominated electric power supply. Therefore, it is essential to replace conventional fossil fuel energy sources with green energy sources, as the number of EVs is growing significantly. The research implies that charging EVs from fossil fuel power supply can not reduce the emission [2].

Additionally, in 2017, the transport sector was producing 27% of the total EU-28 greenhouse gas emission, and the emission from the maritime and aviation are not included. Although the target in Europe is to reduce the emission by two-third in 2050 as a reference of 1990, still the CO₂ released from vehicles raised by 2.2% from 2016 to 2017. The target was set out in 2011 [3]. Hence, replacing fuel cars with EV is a promising option to get a significant reduction in greenhouse emissions, especially EV with vehicle-to-grid (V2G) technology. V2G is the connection of EV and the grid bidirectionally in which power can transfer from the electric network to the vehicle and vice versa. The V2G technology enables the utility to use the EV as backup power by charging during off-peak hours and inject power back to the grid at high demand. The vehicle owners can also get revenue by charging during off-peak and selling the stored energy back to the grid during peak hours. Several countries already implement the bidirectional EV charging system. According to [4], Nissan Leaf is the first carmaker approved by German’s electricity grid to apply the bidirectional integration of the grid and EV, and Nissan uses the CHAdeMO charging standard. Furthermore, the International Energy Agency estimates that 280 million EV will be on the road by 2040. According to the chief strategist in Volkswagen, EV batteries are expected to contribute to stabilizing the grid by injecting power back to the grid when the wind and solar power levels are low or not available. By 2025, it is expected 350 GWh of energy storage from EV fleet, and until 2030 it will raise to 1 TWh. That is greater than the total energy generated by hydropower plants in the world today, and it will open a new business in the electric utility [5].

Denmark can also be observed as an example in 2019; they achieved almost 50% of the electricity source in the country from a wind turbine. It increased by 4% from 2017 to 2019, and the country’s policy is to reduce emissions by 70% under the 1990 level in 2030 [6]. The fast growth of EVs and the implementation of bidirectional power transfer application can improve power system reliability and stability by storing power during surplus in the EV and provide back to the grid when required.

Even if the number of populations in Norway is small, Norway is one of the top EV customer countries in the world, especially electric vehicles are top-rated in Bergen. Bergen ranked as the first city in the world with a significant number of EVs. It reached 20%, or one out of five vehicles in the city is an electric car. The reason to increase the number of electric vehicles in Bergen could be another than the environmental concern. One of the biggest motivations for

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Introduction the people in Bergen for shifting from conventional to the electric car is the discount on road tolls payment for EVs is considerable. However, it is playing a significant role in reducing carbon dioxide emissions. People in Bergen may be motivated by money, but the politicians reach their goal by cutting toll on roads and tax on EVs. Cities like Bergen calls for a bidirectional charging for integrating the EV with the grid to benefit the large number of EVs [7].

1.1 Background

Currently, most of the countries are aiming to achieve zero-carbon dioxide emissions from electric energy production, transportation, and other industries. When it comes to electric energy sources, it is essential for providing environmentally friendly, reliable, and economical electricity for customers. However, it is complex to predict and may not be possible to dispatch some renewable energy sources due to the random variation of their availability. For example, electric supply generating from solar power profoundly affected by the sunshine and the wind turbine also depend on by the blowing of the wind. That means the renewable energies may have a surplus or insufficiency of power generation at certain times. For instance, solar panels may have excess electricity in the day time when it is sunny, and this excess energy will be wasted if the producing energy is more than consuming energy by the load and if there is no storage. On the other side, the production power at night and cloudy weather will reduce significantly, and at this time, the power system requires support from stored energy. However, if there is no storage system, power may need to be generated by non-renewable energy sources. Similarly, the intermittency of wind energy depends on the blowing of the wind. When the wind is blowing, power can be harvested from the wind turbine, and at this time, if there is excess energy, it is required storage to save energy from wasting. On the other hand, there comes a time with no blowing wind results in no energy production, which needs to use from stored energy or other energy sources. Additionally, the loads in the residential, industrial, and commercial fluctuate constantly. Furthermore, the growing number of EVs on the roads induces an additional problem on the power system stability. The EVs are an extra load to the existing electric distribution infrastructure. Therefore, an expensive bulk of batteries is necessary to overcome all the issues stated above. Instead, there is a trend toward using the parked EVs as a distributed energy storage system using V2G technology. The EVs charge when there is low demand in the grid, and discharge to the grid on peak-hours to balance generation and consumption in the grid. EVs with bidirectional connectors decrease the impact of the variability in the production of renewable energy sources by acting as load and distributed energy sources [8].

As the number of EVs growing fast, the deployment of DC fast charging is also increasing to avoid EV customers’ anxiety waiting for charging their vehicle. The DC fast charging draws a high amount of power in a short time, and this causes an additional challenge to the grid.

However, this issue can be changed to positive by bidirectional integration EVs with the grid for providing electric providers and EV owners economic benefit and also solve the technical difficulties in the grid [9]. The two-way power transfer is the concept of the smart grid, and the EVs can respond immediately with a large amount of power if there is a sudden unbalance

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Introduction source is cheapest is a valuable way of saving charging expenses, such as comparing charging at home and commercial charging stations. So, by a little bit of planning and charging as much as possible when the source is cheap, EV owner can reduce charging costs [11].

A vehicle fleet and EVs owned by individuals parked most part of the day, and during this idle time, they are plugged into the grid. So, at this time, the EV battery can provide emergency support to grid stability if there is fluctuation in production or consumption in the power system. For example, it gives frequency regulation in the grid by regulating charging and discharging of real power. In this scenario, the EVs battery is used as a distributed energy resource and load. As the EVs can be considered as energy storage, it is expected to contribute to the increasing installation of environmentally-friendly electric energy sources, like wind and solar power. The smart bidirectional integration of several EVs and the grid is anticipated to play a significant role in renewable energy dominated electric power sources like in Denmark[9].

Presently, EV charging stations are available everywhere. However, the possibility of energy flow is only one way from the grid to the EV battery. Most of the EV companies are working extensively to provide a bidirectional charger. By employing the proper and efficient converters, it is feasible to distribute the two-way power transfer integration of EVs and the grid. According to numerous researches, there are various ways to implement the bidirectional charger. DC fast charging is found to be superior for different reasons. One of the first reasons is charging a vehicle in a short time, and providing sufficient immediate power from the EV to the grid can increase power stability [12]. Implementing this two-way power transfer with smart integration can help to fully electrification of the transport sector to reduce peak hours stress in the distribution part of the grid. Also, EV owners decrease charging costs by contributing power to the grid[9].

There are many parties interested in benefiting from the development of vehicle-to-grid technology. Some of them are vehicle manufacturers, vehicle battery manufacturers, vehicle owners, EVSE owners, the aggregators (service providers), homeowners, and electrical utility.

In residential utilization, vehicle owners, EVSE owners, and homeowners belong to one. When it comes to commercial systems, more participants are probable. The utility has several advantages from V2G technology; two of them are the following. The first is to use EV as storage, and give load-leveling for intermittent renewable energy sources. The second is providing ancillary services for the grid when it needs from the stored energy. The ancillary service is providing power immediately for the grid in case of a power system equipment failure or outage. As mentioned earlier, the adoption of renewable energy sources like solar and wind is increasing, and they are not consistent. The probability of coinciding the peak hours and production time is also less. However, the EV connected with the bidirectional charger plays a significant role in increasing the efficiency of variable renewable energy sources [13].

In a V2G system, informed and a willing vehicle owner is a key to implement the system. The vehicle owners want the car ready whenever they need to drive it. However, the owners should understand the primary contract that they agreed with the electrical utility. Another concern from vehicle owners is participating in grid support can cause battery degradation, and it lowers the life of the battery. However, the design of smart EVSE with programming maintains at a particular state of charge (SOC) can avoid battery depleting. If the participants are well educated about the system V2G technology, they can get significant benefits by employing their vehicle as a battery storage system [13].

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Introduction Furthermore, another entity is required called aggregator in the energy market to obtain successful integration V2G technology economically and technically. The aggregator works as an intermediate between the EVs and electric utility. The aggregator has information about how many EVs are connected to provide power to the grid and state of the grid. The aggregator is responsible for delivering a daily demand forecast for the EV owners to know the best charging time and when there is an energy request from the utility. This information must be approved by the Distribution System Operator (DSO) before providing to the EVs owners[9]

[14].

1.2 Objective and Scope

The foundation for this study is implementing bidirectional power transfer between EV and the grid to get environmental and economic benefits. The tasks of this study are to survey efficient EV battery charging from renewable energy sources, to look over the converter technology for bidirectional power transfer, to review standard for bidirectional EV charging. Lastly, to develop a two-way EV charging simulation models with and without galvanic isolation.

The number of EVs is increasing rapidly, and the electricity generation is also in good trend in the transition to carbon-free energy sources. However, to match the available renewable energy with the demand for charging EVs is crucial to make EVs greener. Different approaches can be used to charge EVs only from carbon-free sources. The fastest-growing renewable energies are wind and solar power, so the EV charging should be managed based on the availability of these sources.

Renewable energy sources are expanding rapidly, and the intermittency of renewable energies cause to raise the importance of the energy storage system. The energy stored in the battery during low demand should discharge back to the grid when the renewable energies are not available. To realize the energy storage and bidirectional power transfer, exploring of converters with the ability of two-way power transfer is a critical factor in the system. A bidirectional AC-DC converter is a fundamental requirement for the energy storage system and two-way power transfer between the EV and the grid. According to the application, different types of converters can be used. A bidirectional DC-DC converter is also essential in some cases for additional voltage regulations. Furthermore, the two-way DC-DC converter classifies in to isolated and non-isolated based on the galvanic separation between the AC source and energy storage battery.

As mentioned earlier, to reduce carbon dioxide emissions to the air, the electrification of the transport sector can play a significant role. The increasing number of EVs can be both challenging and advantage to the power system. By controlling charging patterns, the negative impact of EV on the power system can be reduced. Another vital issue to be considered is the standard connector for charging and discharging EV. Two important reasons to use the standard connector are the connector should be approved by the manufacturer for EV battery health and to comply with the regulation of electric utility when power is injecting from the battery to the grid.

The last objective of this study is to develop simulation models of a bidirectional EV charging

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Introduction converter. The non-isolated converter is a half-bridge (buck/boost), and the isolated converter is a dual active bridge. Only active power is transferred in both directions to charge the EV battery and to maintain the grid frequency stability.

To summarize, the rest of the report is organized as follows: Section 2 presents a survey of a different effective way of charging the EV, while section 3 shows a review of some common converter technologies for bidirectional energy transfer. Part 4 provides the investigation of the available EV charger standards for charging and for bidirectional operation. In parts 5 and 6 are developing a two-way EV charger simulation models with and without galvanic isolation, respectively, in MATLAB/Simulink. Finally, the last section presents the conclusion of all the tasks.

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Efficient EV Battery Charging

2 Efficient EV Battery Charging

Awareness of the impact of a large amount of carbon dioxide emission from the traditional internal combustion engine (ICE) vehicles, prompts to increase the electrification of the transport sector. Charging EV from the conventional energy source produced by fossil fuel will not reduce the emission of carbon dioxide, which is merely switching the emission instead of from the EVs to an energy source power plant. The number of EVs all around the world is growing at an unprecedented rate, and this will increase electric demand. Therefore, it is very timely for both EV owners and electric utilities to find a new way of charging EV from a reliable and environment-friendly energy source to reduce emissions and to save charging costs [15].

2.1 Literature Review

The power consumption by EV is a notably large amount. The power demand of one household in Europe is comparable to Nissan Leaf with a 24kWh battery pack. Consequently, it can increase the current energy demand from 17% to 25% with growth in the number of EVs.

Despite all the positive sides of the EVs in the environment, they can cause a significant and unavoidable negative impact in the power system if the electrification of the transport movement continues in the same trend without considering the power system[16]. As the number of EVs on the roads is increasing, discovering an efficient charging way is required. In this section, an overview of the technical and economically efficient way of charging the EV battery alternatives will be presented.

Installing photovoltaic (PV) is possible in most of the places, including in cities, unlike other power sources, highly dependent on position. As PV is an environmentally friendly source, it can be used to charge EV directly with DC connector. In one research paper [17], they proposed integrating of charging station with a solar power plant with battery storage and grid can provide sustainable power for EV charging off-grid or grid-connected. The approach for this system is charging the vehicle from the PV is the first option, and if there is uncertainty about PV at night or cloudy day, the storage battery takes as a secondary option. During adverse conditions, the grid secures charging at any time. The storage battery should be kept fully charged all the time, and it can also charge from the grid during off-peak hours and use during peak hours to charge the EV if the PV is not available at that time. The system is beneficial both to EV owners economically and technically for the electric utility. The EV owners, by investing in the photovoltaic and extra storage battery, they can save money on charging, and also installing photovoltaic for charging EV reduces the load in the distribution of the power system. Besides, the extra storage battery supports the utility for power system stability by charging during low demand and avoiding connecting during high power demand.

Several numbers of research have been done to explore optimal EV charging strategies, for example, smart charging. The smart charging may define in different ways, but it is a strategy of charging EV by which charging patterns change according to the demand of electrical energy. Three scenarios to apply smart charging are as follows [18]:

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Efficient EV Battery Charging

• Renewable energy scenario: Charging EV when the availability of renewable energy is high as much as possible, like wind and solar.

• Balancing scenario: By varying charging patterns keeping the balance between demand and supply in a power system.

Furthermore, the smart charging can also be classified in two as unidirectional and bidirectional. As mentioned above, with unidirectional charger, it is possible only to charge or disconnect the EV based on the demand in the power system, and the bidirectional charging (V2G) has the additional advantage of exporting power back to the grid from the EV battery.

In the research paper [19], smart charging is defined as intelligent control methods for charging.

The charging control divided into two as centralized (direct) and decentralized (indirect) control. In centralized control, the charging power of EV is decided remotely by the aggregator.

However, in the indirect control method, the EV owners decide in response to the information they receive from the electric utility or aggregator. The charging control is prepared based on the optimization approaches to increase power system stability and satisfy EV owners [19].

San Diego firm Envision Solar is the world’s first company developed completely solar- powered off-grid and mobile EV charging station. They called it Electric Vehicle Autonomous Renewable Charger. Envision Solar International Inc is a San Diego based sustainable technology innovation company. The charger is designed in the size of one car parking, as shown in Figure 2.1, and it can produce around 6000kWh of power per year, and the system has an energy storage of 21.6kWh, which can be used at night [20]. Its mobility and no installation work make attractive this solar charger. The charger produces 18 to 25% more power than fixed solar energy, and it is estimated to charge 150 miles of EV per day, and this distance is far from people they need to drive on a regular day [21]. It doesn’t require great weather for solar power to work. The weather in Germany is rainy, gray and cloudy, but the highest concentration of photovoltaic is in Germany. Accordingly, solar can operate effectively in most parts of the world [22].

Figure 2.1: Stand-alone solar energy EV charging station [20].

The work in [23], proposes installing solar power at the workplace is an excellent strategy to charge EV efficiently. Most of the people work in the daytime, and solar production is maximum in the middle of the day. Furthermore, there is no probability of interruption charging, as most people work continuously for eight hours. The study showed that in this way,

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Efficient EV Battery Charging EV battery charges cost-effective, and it can be injected to the grid when there is an unbalance between supply and demand [23].

The significant development of renewable energy like solar and wind energy provides a possibility to charge EV from the hybrid power plant. The hybrid power arrangement is two or more energy sources that supply power to the charging station. The hybrid infrastructure is more advantageous in rural areas with limitations to access the utility. Some energy sources fluctuate with the weather, but this can cause a problem if the energy source is one. Solar panels produce low energy during a cloudy day or at night, while wind power plants depend on blowing of the wind, and it is windier at night, which is reverse to solar energy. Hence, the probability of exciting at least one source is high, where when one power supply is unavailable, and the other is available. The hybrid infrastructure provides a reliable energy source, and the availability of power anytime can realize efficient charging of EV, and as a result, the distribution of EVs increase [15] [24].

A wind-solar power station developed by Sweden-based energy company called Innoventum can charge EV, power home, and it can also connect to the grid. It is made up of a wooden structure to hold 24 solar panels and a wind turbine attached at a 12-meter height pole, as shown in Figure 2.2, and its name is called Giraffe 2.0. The hybrid power station can produce 13.8 MWh per year and almost 38 kWh per day. The combination of solar and wind makes it reliable, so the system is expected to provide electricity consistently, but the amount of generation may vary from time to time. The system can be used for different applications, and if it is used only for charging EV only, it can supply an EV charging station with a 50 kW DC- fast charger or two level-2 charging connectors. Even if the system is expensive, about 55,000 Euro, it is a useful solution for remote areas that have no access to the grid. The Giraffe 2.0 system is already installed in Sweden and other parts of the world [25].

Figure 2.2: Giraffe 2.0 hybrid (solar-wind) stand-alone power supply system [25].

In the past several years, the price of solar panels has dropped, intending significant growth in

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Efficient EV Battery Charging silicon in the photovoltaic cells. Now, the thickness is 160 micrometers, and this can be lowered to 100 micrometers still works efficiently. This reduction is possible to be done by the available technology, but a further decrease in thickness may require new development in technology.

This reduction plays a vital role in price reduction and the distribution of the manufacturing of solar panels. Another research [27] shows that traditional silicon solar cells can be replaced by cadmium telluride (CdTe). This material is similar to a silicon solar cell in its efficiency, but cadmium telluride operates better at hot and cloudy weather. Also, CdTe is 10% cheaper than silicon solar in the present market. According to these researchers, the charging of EV from a solar power plant will be efficient, and it will play a significant role in reducing the stress in the distribution part of the power system due to the increased number of EVs with bidirectional connector.

For people homeowners, by investing in installing renewable sources such as solar and wind power at home, they can avoid EV charging costs. Even if the power is available when it is shine or windy, by charging during the excess power available and discharge to the grid by connecting with a bidirectional charger in the charging station, the utility will pay them for the contribution. Indeed, charging EV from solar or wind is the most cost-effective and environmentally friendly nowadays [11].

By installing solar panels on the roof at home, power can be fed to the grid during the production of excess energy more than all the household appliance consumes. However, the revenue to supply power to the utility is not satisfactory in several countries, and there are some technical criteria to comply with for providing electrical power to the grid. The technical requirements are voltage regulations, power factor response to frequency change or short circuit in the grid, reactive power supply, and also the amount of power exporting. Instead, it would be better to store the excess energy in the EV when the sun is shinny to use it when the sunsets to power home. The modern EV battery is good enough to power all the households for several days. The EV battery is operating as energy storage, and the concept is the same as V2G, which is called vehicle-to-home (V2H) [28].

V2G system is an EV charger with bidirectional power transfer functionality. In the Netherlands province Utrecht, a world-first smart solar power charging station for EVs with the technology of V2G tested in 2015. Twenty-two charging stations with the capability of charging and discharging installed throughout the province and almost 40 EVs participated in the system. The EV owners were told to charge their cars when the solar is producing abundant, and they have to connect with a bidirectional charger to the grid to supply the unused energy when renewable energy is not available. Each customer was able to save an average of 131 to 820 Euros per year. Based on the result, the program was satisfactory, and they announced that 1000 additional chargers with V2G functionality and 10,000 solar panels are expected to be installed [29].

2.2 Discussion

The literature reviewed in this section helped to investigate the latest trends in technology emerging recently for effective charging of EV from environmentally friendly energy sources.

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Efficient EV Battery Charging EVs become green and eco-friendly only if they charge from the emission-free energy source.

The several approaches used to make EV charging economical and environmentally friendly are discussed here.

EV charging station integrated with solar power, additional storage battery, and the grid can provide sustainable, eco-friendly, and cost-effective electricity for charging the vehicle. The system applies to commercial or public charging stations to ensure power system stability and reliability. Also, if the system is installed with V2G bidirectional capability in the workplace, it would be more beneficial for both customers and electric utility. The workplace is the longest time people can park their cars outside their homes. However, this application is expensive for individuals to implement in residential due to the extra cost of the photovoltaic and storage battery.

Another essential consideration is that to establish a managed charging system when renewable energy is not off-grid. If the grid is not entirely renewable energy, to develop a system encouraging customers to shift charging times and offering a special discount at a time with abundant renewable energy generation is essential to balance supply and demand. For example, solar is ample during particular daytime, and the wind is abundant at night. The system can be implemented to charge the vehicle either when high renewable energy generation or during off-peak hours.

EV can be used well beyond the means of transport as battery storage for intermittent energy sources such as wind and solar. The main advantages of the bidirectional V2G application are the following: The variability of renewable energy sources can be improved by using the EV battery as a battery storage system and discharge to the grid when it is required. Generation and load balancing in a power system are possible by valley fillings and peak load shaving.

Valley filling, in this case, is charging EV at off-peak hours like at night and avoid charging during peak hours and provide power from EV to the grid when it is possible. Peak load shaving is stopping charging EV quickly if the power consumption spike, which is also called load shedding. If all these are correctly applied, it gives economic benefit for EV owners and electricity providers.

Charging stations powered only from renewable energy like Giraffe 2.0 is power from solar and wind, and San Diego firm Envision Solar are one of the best ways of charging EV regardless of the initial price.

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Bidirectional Converter Technology

3 Bidirectional Converter Technology

According to several researchers, the traditional converter topologies for EV chargers have only one stage front-end AC-DC converter, and the DC fast charging includes back-end DC- DC converter for better voltage regulation [30]. Similarly, the bidirectional EV charger has two interfaces of power converters to provide the possibility of connecting a variety of vehicles with different voltage levels with one connector. The first stage operates as a rectifier during battery charging. Alternately, it works as an inverter when power transfer from the battery pack to the AC mains. The second stage is the DC-DC converter step down the voltage to the required voltage level for the EV battery during charging and boost the voltage supplied to voltage source inverter during power sourcing to the grid.

Recently the bidirectional power flow from the grid to EV battery and the other way around is getting more attention. The bidirectional converters are the main foundation for vehicles to contribute as a mobile energy storage system. By utilizing the charger with the ability to transfer the power two-way, the EV can be used well beyond the means of transport as battery storage for intermittent energy sources, which charges the EV battery and discharge to the grid with the same connection. To achieve this bidirectional power integration, it requires intelligent and complicated converters with an advanced control system [10]. The EVs are employed as a distributed battery energy storage system (BESS) in the power system. To realize these converters, among the essential considerations, are size, weight, power density, reliability, efficiency, cost, control type of the converter, and so on [31].

With the advancement of semiconductor technologies, researchers have been working extensively on bidirectional converters. The topology of converters can generally be full-bridge or half-bridge. Half-bridge converters are characterized by a few switching components, lower cost, and excellent performance, although there is stress in the elements. Conversely, in full- bridge converters, the price rises with the number of components increase, but the stress in the devices decreases [31]. Metal-Oxide-Semiconductor-Field-Effect-Transistors (MOSFETs) and Insulated Bipolar-Transistors (IGBTs) with anti-parallel diode are the most famous in the power switching converter because of the ability of bidirectional conduction, and higher frequency capability. The converters used in the EV charger for two-way power transfer are bidirectional AC-DC and DC-DC converters. This part reviews the leading converter technology used in the application of V2G technology [32].

3.1 Bidirectional AC-DC converters

The power transfer in the bidirectional AC-DC converter classified into two modes of operation. When the conversion is from AC to DC, it is known as rectifier mode, and during DC to AC conversion, it operates as an inverter. Both the rectifier and inverter mode are used in unidirectional power flow application. The current flow in both directions realizes the two- way power transferring in the converter. Either IGBTs or MOSFETs can be implemented in switching the converter based on the applications to ensure the bidirectional conduction.

Historically, the grid-connected bidirectional converter is developed from the front-end PWM rectifier to supply power from the mains to DC machine drives. Conversely, during regenerative braking, the inverter mode is used to provide the power back to the grid instead of wasting it. Both rectifier and inverter mode have the same control method, PWM. However, when power is to be injected into the grid from regenerative braking or other renewable

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Bidirectional Converter Technology energies, it requires better power quality to provide the utility by applying such as filters and a better control system. The control of the AC-DC converter should operate in such a way that the phase voltage of the power system should not be affected by charging the EV or providing back power to the grid from the EV battery. Converters with diodes and thyristors introduce harmonics to the grid, but converters controlled with PWM increase the power factor result in raising real power transfer and improve the current harmonics [33]. The bidirectional AC-DC converter can be implemented with a single-phase and three-phase.

3.1.1 Single-Phase Bidirectional AC-DC converter

Single-phase ac-dc bidirectional converter is broadly used in different applications, such as load connected to the grid and distributed energy resources (DERs), EV connected with bidirectional charger, and uninterruptible power supply (UPS). When the AC-DC converter is integrated with the grid, there are several requirements to be satisfied like low distortion line current, power factor correction factor, high-quality dc output voltage, and efficient bidirectional power transfer capability [34]. In single-phase AC-DC converter, half-bridge, and full-bridge topology can be used. The half-bridge scheme experiences higher components stress, but fewer components and lower cost [30].

In [34], the proposed single-phase AC-DC bidirectional converter is deployed in a DC load to interface the grid, and the load is connected directly to the distributed energy resource (DER), as shown in Figure 3.1. The microgrid is dc source, and if there is a surplus in the DER, the converted works as an inverter to supply energy to the utility. Conversely, when power is lacking in the DER, the converter rectifies to provide power the load from the grid.

Figure 3.1: A bidirectional single-phase ac-dc converter in a load connected to dc microgrid and the grid [34]

A simplified PWM strategy is employed for switching the converter in Figure 3.1. The simplified PMW approach uses one-fourth of the traditional bipolar PWM and unipolar PWM in the number of switching. During the switching event in the simplified PWM switching, it needs only one switch to change the status. Whereas, in the unipolar PWM or bipolar PWM demands four switches to change the state during the switching time. Consequently, the switching losses reduce significantly in the simplified PWM switching, and it has higher conversion performance. Also, a feedforward control strategy is used to switch between the inverter mode and rectifier mode shown in Figure 3.2. The simplified PWM switching method has higher efficiency than bipolar PWM and unipolar PWM, and lower total harmonic distortion than bipolar PWM [34].

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Figure 3.2: Control scheme for the simplified PWM switching [34].

3.1.2 Three-Phase Bidirectional AC-DC converter

Similar to the single-phase, a three-phase AC-DC converter is used in different applications in addition to bidirectional EV charger, such as uninterruptible power supplies, motor drives, grid- connected renewable energy, and battery storage system [33]. The bidirectional power transfer V2G has become essential, particularly in the off-board charging. Very often, the off-board charging station is connected to three-phase power. For an effective bidirectional electric energy transfer between the EV and the grid, a three-phase two-way AC-DC converter is selected as it can handle higher power than a single-phase converter. The higher power handling is essential for quick charging and fast response when the grid needs support from the EV battery with a bidirectional charger. Also, the EV battery is accessed directly by the off- board charging because the converter is in the charging station outside the vehicle. However, the converter is inside the car for an on-board charger. There are several three-phase topologies used for bidirectional converters. Hence, the investigating of different available converters is very crucial to get an efficient converter. Among the required factors are power quality with low THD, high power factor, a low ripple in DC side current, and low distortion in the AC voltage and current [35].

If power MOSFET is used in a three-phase converter, it can cause converter failure due to the inherent MOSFET's body diode conduction. IGBT also has higher conduction and switching losses than MOSFET. Besides, IGBT operates at a lower switching frequency comparing with MOSFET. However, with the evolution of technology, power MOSFET has been used in high power transfer by making the MOSFET’s drain-source on-resistance (𝑅_{𝐷𝑆(𝑜𝑛)}) extremely low.

Hence, switching applications with the MOSFET has very low conduction losses, small switching losses, and higher efficiency [33].

For grid-connected converter bidirectionally, it is assumed that the phase voltage and current of the grid are not influenced by the power transfer to or from the battery. The utility time- varying phase voltages for the three-phase system for the given RMS phase voltage (𝑉_𝑟𝑚𝑠), frequency (f), and time (t) are expressed as follows below from equation (3.1) to (3.3) [10].

𝑣_𝑎(𝑡) = √2𝑉_𝑟𝑚𝑠𝑐𝑜𝑠⁡(2𝜋𝑓𝑡) (3.1)

𝑣_𝑏(𝑡) = √2𝑉_𝑟𝑚𝑠𝑐𝑜𝑠⁡ (2𝜋𝑓𝑡 −2𝜋

3) (3.2)

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𝑣_𝑐(𝑡) = √2𝑉_𝑟𝑚𝑠𝑐𝑜𝑠⁡ (2𝜋𝑓𝑡 +2𝜋

3) (3.3)

The time-varying phase currents for the three-phase are also expressed as follows below with a phase angle shifted 𝜑_𝑖 from the voltage.

𝑖_𝑎(𝑡) = √2𝐼_𝑟𝑚𝑠𝑐𝑜𝑠⁡(2𝜋𝑓𝑡 + 𝜑_𝑖) (3.4)

𝑖_𝑏(𝑡) = √2𝐼_𝑟𝑚𝑠𝑐𝑜𝑠⁡ (2𝜋𝑓𝑡 −2𝜋

3 + 𝜑_𝑖) (3.5)

𝑖_𝑐(𝑡) = √2𝐼_𝑟𝑚𝑠𝑐𝑜𝑠⁡ (2𝜋𝑓𝑡 +2𝜋

3 + 𝜑_𝑖) (3.6)

Where:

𝑉_𝑟𝑚𝑠 RMS utility phase voltage [V]

𝐼_𝑟𝑚𝑠 RMS utility phase current[A]

f frequency [Hz]

t time [s]

It is clear from the voltage and current showed above, the real power P provided to the grid is given in watts W, and the reactive power is given in volt-amper reactive (Var).

𝑃 = 𝑣_𝑎𝑖_𝑎+ 𝑣_𝑏𝑖_𝑏+ 𝑣_𝑐𝑖_𝑐 = 3𝐼_𝑟𝑚𝑠𝑉_𝑟𝑚𝑠cos⁡(𝜑_𝑖) (3.7)

𝑄 = 3𝐼_𝑟𝑚𝑠𝑉_𝑟𝑚𝑠sin⁡(𝜑_𝑖) (3.8) The apparent power S in volt-amperes is also expressed as:

|𝑠| = (𝑃² + 𝑄²)^1/2 = 3𝐼_𝑟𝑚𝑠𝑉_𝑟𝑚𝑠 (3.9)

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Bidirectional Converter Technology providing discharging power from the battery or decrease by charging the battery using the bidirectional charger. The reactive power is not necessary to provide by EV battery if the grid requires reactive power support for voltage regulation in an inductive load. The capacitance in the DC-link is large enough to provide Var [10].

The phase angle 𝜑_𝑖^∗ and the efficiency of the converter is expressed as:

𝜑_𝑖^∗ = 𝑡𝑎𝑛⁻¹(𝑄^∗

𝑃^∗) (3.10)

𝜂 = 𝑃

𝑖_𝐷𝐶𝑣_𝐷𝐶 (3.11)

The equation (3.11) determines the efficiency of the bidirectional AC-DC converter during inverter mode, and it is clear from equation (3.10) that when the phase angle, 𝜑_𝑖 between voltage and current is 0° or 180°, for transferring only real power.

Where:

𝑃 real power in the AC side of the converter [W]

𝑃^∗ reference real power [W]

Q reactive power in the AC side of the converter [Var]

𝑄^∗ reference reactive power [Var]

𝜑_𝑖^∗ reference phase angle [ radian]

𝑖_𝐷𝐶 steady-state current in the DC side of AC-DC converter [A]

𝑣_𝐷𝐶 voltage in the DC side of AC-DC converter [V]

S apparent power [VA]

A three-phase two-level H-bridge bidirectional AC-DC converter is an elementary and simplified topology in the three-phase converter, which is shown in Figure 3.3. This converter has a simple control arrangement and cheap components. However, the current flow concentrates on a few devices, which lead to concentrated losses either in the top or bottom switches. The converter uses IGBTs when the voltage reaches up to 1200 V range, and MOSFETs are also popular in this topology [36].

Figure 3.3: Three-phase two-level H-bridge bidirectional three-phase AC-DC converter [38].

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Bidirectional Converter Technology Three-phase four-leg converter is presented in [33], as shown in Figure 3.4. The neutral line is used to balance the three-phase outputs. The size of the capacitor and the passive components are smaller than the two-level H-bridge converter in this topology. However, as the number of switches increase, the control gets complicated.

Figure 3.4: Three-phase four-leg bidirectional AC-DC converter [33].

Another topology of the bidirectional AC-DC converter is a three-phase three-level neutral point clamp (NPC). As can be seen in Figure 3.5, multiple switches arranged one over another that benefits to fold voltage link, which decreases the importance of filter. In this topology, low rating switches can be used to give the same operation with the two-level H-bridge converter.

It is clear from the structure that to control this converter is more complicated than the two- level H-bridge converter shown in Figure 3.3 due to the increased number of switches.

However, the increased level has advantages to reduce total harmonic distortion, gives higher power quality, ripple-free, and regulated DC voltage output is not aggressive to supply and load disturbances [30] [37].

Figure 3.5: Three-phase three-level neutral point clamp AC-DC converter [30].

A single-stage bidirectional AC-DC converter can be used in a two-way power transfer to connect the battery with the grid. The DC link of the converter is directly connected with the battery. Still, the capacitor should be large enough to provide a constant voltage for the battery

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Bidirectional Converter Technology topology is the switches conduct when the gate is off. The transition between on and off causes considerable losses due to high switching losses in IGBT [10].

Figure 3.6: Single-stage three-phase bidirectional AC-DC converter [10].

3.2 Bidirectional DC-DC converter

A DC-DC converter is an electronic circuit used in different applications to change the voltage from one level to another in a DC form. With increasing the demand for renewable energy, the importance of a bidirectional DC-DC converter is expanding as the implementation of the batteries for storing energy, and sourcing energy to the grid is required. The DC-DC converter is necessary for the two-way power transfer for additional voltage regulation during the battery charging/discharging, which provides flexible voltage regulation between EV battery and the DC-bus. In some applications, further regulation is not significant. However, in the bidirectional EV charger, both bidirectional DC-DC and AC-DC converters are required [10].

There are many applications of bidirectional DC-DC converters, such as battery charger, hybrid EV systems, UPS, fuel-cell hybrid system, PV hybrid power system. Basically, the bidirectional DC-DC converters are classified into two, according to the galvanic isolation between the battery and the grid [38]. These are:

• Non-isolated bidirectional DC-DC converter

• Isolated bidirectional DC-DC converter

3.2.1 Non-isolated bidirectional DC-DC converter

Non-isolated bidirectional DC-DC converters are employed when size and weight are to be considered, such as in aircraft power systems. This converter is simple and has fewer components than isolated bidirectional DC-DC converters [38]. Also, due to better efficiency and lower cost, the non-isolated converter is preferred than the isolated converter [33]. There are several non-isolated bidirectional DC-DC converters according to findings in different researches work. However, the famous and widely employed converter is half-bridge, and it operates in boost mode in one direction and buck mode in the other direction [39]. The transformerless non-isolated converter topology is more widely used than the isolated converter [37].

In the research papers [40] [41], cascade buck-boost DC-DC converter topology is proposed for bidirectional energy transfer. It operates as a buck converter while charging the battery, and as a boost converter when the power is delivering back to the grid to provide the inverter at a higher voltage. The input and output voltage levels differ from vehicle to vehicle. The buck- boost converter is used to achieve each vehicle requirement. The intermediate capacitor stores

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Bidirectional Converter Technology energy at a higher voltage to overlap the DC bus voltage and the battery voltage, as shown in Figure 3.7. Besides, this intermediate capacitor voltage can be altered to enhance the transient performance of the converter. The two stages separated by the capacitor give a better battery voltage capability, but higher power losses due to the increased number of switches. Generally, the cascade buck-boost converter has low efficiency, and performance falls when the voltage transfer degree rises.

Figure 3.7: Cascade bidirectional DC-DC converter topology[40].

The half-bridge bidirectional DC-DC converter shown in Figure 3.9 is proposed in [42]. This topology needs two switches that can operate bidirectionally such as MOSFETs and IGBTs, and one inductor in the battery side to store energy during the charging and discharging of the battery. The bidirectional half-bridge DC-DC converter is derived from the basic buck and boost converters. The only buck and boost converters are not capable of power transfer bidirectionally due to the property of diode in the circuit, see Figure 3.8. The diode conducts only in one direction when it is forward biased.

V in

L L

C

RL

D

C

(a) Buck converter (b) Boost converter

Vo

Vin

+

- -

+

Figure 3.8: Unidirectional buck and boost converters.

To operate the buck-boost converter bidirectionally, replace the diode with controllable switch conducting in both directions like MOSFET or IGBT with an anti-parallel diode across them to allow two-way power flow. Figure 3.9 operates as a buck mode when power transfers from High voltage (HV) to low voltage (LV), and as boost mode during the reverse power transfer.

On page 42, in the modeling of non-isolated bidirectional EV charger, this bidirectional buck- boost DC-DC converter is used to regulate the voltage level in the DC side. The reduced number of components, low cost, lightweight, and high efficiency make superior this topology [38].

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Figure 3.9: Buck-boost bidirectional DC-DC converter [42].

Another a three-level bidirectional DC-DC converter is proposed for a DC fast EV charging station in the research articles [39] [43]. The three-level topology is used in high voltage and high power rating applications, and it is known by less voltage stress across the switches. Also, it requires smaller passive components of inductors and capacitors. As shown in Figure 3.10, the three connection points p, n, z are connected to the DC-bus charging station, and the other end of the converter is connected to the battery. The modulation process generates the switching pulse, and by implementing a new modulation technique, the switching frequency can be reduced. Due to the reduced switching frequency, the losses across the switches can be decreased in this topology. The operation mode of the three-level bidirectional DC-DC converter is similar to the half-bridge converter, operates as boost mode during battery discharging and buck mode when the battery is charging.

Figure 3.10: Bidirectional three-level DC-DC converter [43].

3.2.2 Isolated Bidirectional DC-DC converter

An isolated DC-DC converter is employed when it requires output and input to be separated to provide safety. In several applications, galvanic separation using the isolated converter is essential. The non-isolated topologies are convenient to use when the factor between input and output is small. However, applications with output voltage differ from the input voltage by a significant factor convenient to use an isolated converter. Hence, a high-frequency transformer must be added between the input and output stages for galvanic isolation, and a converter with high-frequency switching with bidirectional operation also requires. Half-bridge, full-bridge, and push-pull or center-tapped converters are widely used topologies. The transformer provides the galvanic separation and voltage matching between the mains and energy storage batteries in the bidirectional DC-DC converter. Besides, it gives multi-output connections and reduction of stress during switching operations [44]. The voltage level is adjusted by modifying the turns

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Bidirectional Converter Technology ratio of the transformer [38]. The transformer raises the size, losses, noise, and price of the converter. Even if the bidirectional isolated DC-DC converter is more expensive due to the increased devices, but it provides higher power density and fast control [30]. Some popular bidirectional DC-DC converters are reviewed below.

A bidirectional isolated dual-active-bridge (full-bridge) DC-DC converter is presented in [41]

[45], as shown in Figure 3.11. 𝐶₁ in Figure 3.11 is connected to DC bus and the other end is coupled to the storage battery. Its galvanic separation is by a high-frequency transformer, which is smaller in size than the conventional transformer. The function of the bidirectional AC-DC bridge on both sides is to provide AC power to the high-frequency transformer and supplies DC to both ends of the converter. The DC voltage in 𝐶₁ is converted to AC to transfer the other side of the transformer in AC form and then converted to DC to store in the battery. Similarly, during power transfer from the battery, the battery DC voltage converted to AC then transferred to the other side of the transformer again convert to DC. The amount of power and its direction is controlled by the phase shift angle to achieve zero-voltage switching. The switching control can be done in several ways, and two of them are the following. Either one bridge is phase shift controlled, and the other is uncontrolled (only anti-parallel diodes conduct), or both bridges output a square voltage wave, and the phase between two voltages square waveform can be controlled. The latter one is the most popular and less complex, which is called a single phase-shift method. The polarity of the phase shift angle controls direction power flow, and the magnitude of the phase shift angle controls the amount of power transfer. Managing the amount and direction of power transfer using only one variable makes simple the single-phase shift technique [46]. This on-board converter shown Figure 3.11 is characterized by high power density, low harmonic distortion, unity power factor, excellent reliability. Besides, other advantages of the dual active bridge converter are soft-switching property, the evenly distributed current in the switches, and the few numbers of passive devices [47]. The dual- active-bridge topology operates efficiently if the ratio of the DC-voltage in the high voltage side and low voltage side is equal or close to the transformer turns ratio [48] [49]. In this topology, the capacitor in the storage sides of the DC-DC converter is tiny, and this makes it less costly, unlike the non-isolated Cascade DC-DC converter topology in Figure 3.7 used expensive, bulky capacitor [41]. The drawbacks of this converter are the narrow voltage range for best operation, and difficult to achieve high efficiency during light load due to high ratio reactive power to active power [45].

Figure 3.11: Bidirectional full-bridge DC-DC converter based on two voltage-fed [45].